This invention relates to a method for detecting the volume of the droplets of ink ejected by a thermal ink jet printhead and to an ink jet printer, operating in accordance with said method, having the ability to automatically set optimal printing modes.
At present, both the printers based on ink jet technology and also the printheads used on these printers possess considerable integration between their constituent elements, for the purpose of obtaining the best results in terms of printing quality and operating reliability.
Unfortunately, even with a highly integrated construction and despite various manufacturing stratagems, ink jet printers and relative thermal printheads in actual fact come with dimensional shape errors, albeit minimal, with respect to a nominal condition and also differences from one article to the next, which may impinge, sometimes significantly, on the performances obtained from them and on printing quality in particular.
This drawback is normally due to the errors, tolerances, and dispersion typical of the manufacturing and/or assembly cycle via which the various parts comprising an ink jet printer and relative printheads are made and assembled.
This is especially true of the ink jet printhead arranged for ejecting the droplets of ink in each printer, which is constructed in a very complex manufacturing process consisting of numerous steps and the integration of many components.
In addition, extremely stringent economic criteria which must be satisfied by most of the currently marketed ink jet head models, particularly the xe2x80x9cdisposablexe2x80x9d ones, do not for cost reasons allow each printhead produced to be checked individually, nor any deviation found of the printheads from a nominal condition to be eliminated.
Likewise, the taking of action through continuous adjustments of the printhead manufacturing cycle is almost impossible so that in the final analysis the latter, in actual fact, always come with a certain range of dispersion, even if normally accepted, of their characteristics and in particular of their dimensional parameters.
In general, the factors that may condition, as a result of errors with respect to the nominal conditions and/or of reciprocal interactions, both the reliability and also the final print quality obtainable with an ink jet printer, are numerous, and some of these are listed below for clarity""s sake:
the firmware resident on the ink jet printer, namely the special program for each printer model, which is adapted to manage some basic operations during printing and which in particular defines the timing of the ink jet head driving;
the ink jet head driving circuit, namely the circuit intended for directly controlling the printhead by supplying it the energy necessary for ejecting the droplets, and which typically comprises a power supply and a plurality of driving components, arranged on board both the printer and the printhead;
the volume of the droplets ejected by the head, which determines the size of the printed dot;
the printer driver, namely the program, normally installed on the computer connected to the printer and cooperating with the firmware resident on the latter, which processes the original image, dot by dot, in order to convert its chromatic data into correct commands for the printer, so that the latter performs printing of the original image on a print medium, such as a sheet of paper. In particular, the printer driver operates on the chromatic data of the image depending on various parameters, among which the size of the elementary dot of the image to be printed, the type of print medium, etc., and incorporates suitable algorithms of diffusion of the graphic errors so as to optimally control the printer and accordingly obtain the best print quality.
The general concept of keeping the volume of the droplets ejected by a thermal ink jet printhead under control, in order to improve the performances and final print quality obtainable with the printhead, has been known in the sector art for some time.
For example, the U.S. Pat. No. 5,036,337 describes a method intended for maintaining the volume of the droplets ejected by a thermal ink jet printhead in accordance with a desired value over time.
In this method, an indicative table of reference of the performances obtainable with the ink jet printhead is predefined in advance in empirical fashion, by way of experimental surveys carried out on a wide range of thermal ink jet printheads produced, so as to take into account the tolerances and dispersions typical of their manufacturing process. The reference table is then polled during the printing step so as to condition, through a control circuit, the times and characteristics of the pulses that drive the actuating resistors of the printhead to determine ejection of the droplets.
This method is limited by being based on numerical reference data that are fixed and defined a priori, instead of information continuously updated in real time, indicative of the actual progress of the printing process.
A method is also known from the U.S. Pat. No. 5,767,872 filed on behalf of the Applicant for automatically setting the optimal energetic working point of a thermal ink jet head, that is to say the optimal value for the driving energy to be sent to the ejection resistors of the printhead in order to guarantee a stable ejection of droplets, with a substantially constant volume. This method comprises a test starting cycle during which the ejection resistors of the ink jet printhead are driven with a variable driving energy, for the purpose of experimentally detecting a critical value for the driving energy corresponding to an operating condition of the printhead on the borderline between a zone of unstable emission, at variable volume, of the droplets, and that of stable emission, at a substantially constant volume, of the droplets.
The method then calculates and sets automatically, on the basis of the critical driving energy value detected previously and in particular by incrementing this critical value according to a predetermined percentage, an optimal value for the driving energy with which to drive the resistors in nominal operation. In this way, a nominal operation of each printhead is guaranteed that is undoubtedly inside the zone of stable emission of the droplets, despite the manufacturing tolerances and the lack of precision of the different printheads.
The method has the distinct advantage of giving an effective and automatic setting for each thermal ink jet printhead, making allowance for manufacturing tolerances, in such a way as to permanently obtain a stable emission of droplets; however, it also has the drawback of ignoring, at least in part, the importance of the parameter that is the actual volume of the droplets of ink ejected for constantly guaranteeing optimal print quality. Besides, in particular, this method gives no indication as to how this actual volume of droplets ejected can be determined.
Another known method, disclosed by document U.S. Pat. No. 5,682,183 and provided for determining imminent ink exhaustion in a thermal inkjet print cartridge, is based on the discovery that ink drop volume falls at a faster rate at high frequency firing rates than at low frequency firing rates, as ink supply diminishes. The method includes warming the print cartridge printed and ink to a predetermined temperature; then operating the print cartridge printed at a first firing frequency to eject a volume of ink, said operating step including heating the ink and printed, carrying away heat in the ejected volume of ink, and conveying a volume of cooler ink to the printed to replace the ejected volume; and monitoring a first temperature change from the predetermined temperature. Then warming the same print cartridge printed and ink to a predetermined temperature; operating the print cartridge printed at a second firing frequency which is different than the first firing frequency to eject a volume of ink in the form of droplets, said operating step including heating the ink and printed, carrying away heat in the ejected volume of ink, and conveying a volume of cooler ink to the printed to replace the ejected volume; and monitoring a second temperature change from the predetermined temperature. The first and second temperature changes are compared to indicate a low ink supply. However also this method is not capable of giving indication as to how the actual volume of the ejected droplets can be determined.
The primary object of this invention is to define a method for detecting in a sufficiently reliable and precise way the actual volume of the droplets ejected by a thermal ink jet printhead, in order to permit a more effective control and use of this important parameter in ink jet printing.
Another object of this invention is to define a method permitting to significantly improve the performances, particularly printing quality, obtainable from a printer provided with an ink jet printhead, based on detection of the volume of the droplets of ink ejected by the ink jet printhead.
The above objects may be attained by means of a method and device for automatically detecting the volume of the droplets ejected by a thermal ink jet head, having respectively the steps and characteristics defined in the main independent claims.
In particular, according to what is demonstrated by this invention, the detection of the volume of the droplets ejected by a thermal ink jet printhead is used to set automatically, i.e. without any intervention from a user, the printing modes during operation of the printer in which the printhead is fitted, so as to constantly optimize the printing quality obtained.